Back to EveryPatent.com
United States Patent |
6,156,392
|
Duffy
,   et al.
|
December 5, 2000
|
Process for triboelectric application of a fluoropolymer coating to a
threaded fastener
Abstract
The present invention is directed to a process for the application of
fluoropolymer to a preselected area of a threaded fastener, and
particularly to substantially all of the threads of the fastener. The
fluoropolymer is supplied to a spray nozzle in powder form and is
subjected to a triboelectrostatic charging process so that individual
particles discharged from the spray nozzle are electrically charged. In
the preferred form of the invention, the fluoropolymer powder is
triboelectrically charged, entrained in an air stream discharged from the
nozzle and directed onto the preselected area of the fastener. In this
manner a generally uniform powder coating is deposited onto the
preselected area of the fastener while the fastener is maintained at room
temperature. Thereafter, the fastener is heated to a temperature above the
melting point of the fluoropolymer to thereby coalesce the deposited
powder into a continuous film coating which adheres, upon cooling, to the
pre-selected area of the fastener. In accordance with a preferred
embodiment, the fastener is heated in a manner which raises the
temperature of only the preselected area of the fastener to the
fluoropolymer melting point. This preferred heating technique minimizes
the retention of fluoropolymer inadvertently deposited on areas of the
fastener other than the preselected area, and allows this undesired
fluoropolymer to be easily removed, even after heating.
Inventors:
|
Duffy; Richard J. (Shelby Township, MI);
Sessa; Eugene D. (Harrison Township, MI)
|
Assignee:
|
Nylok Fastener Corporation (Macomb, MI)
|
Appl. No.:
|
352431 |
Filed:
|
July 13, 1999 |
Current U.S. Class: |
427/475; 427/476; 427/477; 427/481; 427/486 |
Intern'l Class: |
B05D 001/06; B05D 007/22 |
Field of Search: |
427/475,476,477,481,485,486
|
References Cited
U.S. Patent Documents
Re33766 | Dec., 1991 | Duffy et al.
| |
1751343 | Mar., 1930 | Mack.
| |
1835603 | Dec., 1931 | Kincaid, Jr.
| |
2761997 | Sep., 1956 | Forest et al.
| |
2764712 | Sep., 1956 | Juvinall.
| |
2770494 | Nov., 1956 | Nelson.
| |
2806444 | Sep., 1957 | Werner et al.
| |
2808343 | Oct., 1957 | Simmons.
| |
2893894 | Jul., 1959 | Ransburg.
| |
2903190 | Sep., 1959 | LeDeit.
| |
2961581 | Nov., 1960 | Grave et al.
| |
3021077 | Feb., 1962 | Gauthier.
| |
3022016 | Feb., 1962 | Shrewsbury.
| |
3048498 | Aug., 1962 | Juvinall et al.
| |
3069387 | Dec., 1962 | Allen.
| |
3082956 | Mar., 1963 | Point.
| |
3085749 | Apr., 1963 | Schweitzer et al.
| |
3099497 | Jul., 1963 | Albert.
| |
3121024 | Feb., 1964 | Wampler et al.
| |
3147146 | Sep., 1964 | Sedlacsik, Jr.
| |
3178114 | Apr., 1965 | Point.
| |
3248606 | Apr., 1966 | Fraser.
| |
3335965 | Aug., 1967 | Riccio.
| |
3441073 | Apr., 1969 | Johnson.
| |
3494243 | Feb., 1970 | Kleinhenn.
| |
3579684 | May., 1971 | Duffy.
| |
3720533 | Mar., 1973 | Gallagher.
| |
3723933 | Mar., 1973 | Loepfe et al.
| |
3787222 | Jan., 1974 | Duffy.
| |
3814156 | Jun., 1974 | Bachmann et al.
| |
3894509 | Jul., 1975 | Duffy et al.
| |
3896760 | Jul., 1975 | Duffy.
| |
3903321 | Sep., 1975 | Schaad.
| |
3975787 | Aug., 1976 | Newnom.
| |
4000982 | Jan., 1977 | Ueda.
| |
4035859 | Jul., 1977 | Newnom.
| |
4060868 | Dec., 1977 | Axvig et al.
| |
4071192 | Jan., 1978 | Myers.
| |
4080233 | Mar., 1978 | McCloskey et al.
| |
4100882 | Jul., 1978 | Duffy et al.
| |
4109027 | Aug., 1978 | Crose.
| |
4114505 | Sep., 1978 | Loeser et al.
| |
4114564 | Sep., 1978 | Probst.
| |
4120993 | Oct., 1978 | Duffy et al.
| |
4154871 | May., 1979 | White et al.
| |
4285378 | Aug., 1981 | Wallace.
| |
4366190 | Dec., 1982 | Rodden et al.
| |
4380039 | Apr., 1983 | King.
| |
4626365 | Dec., 1986 | Mori.
| |
4656051 | Apr., 1987 | Wojcik.
| |
4689241 | Aug., 1987 | Richart et al.
| |
4775555 | Oct., 1988 | Duffy.
| |
4779558 | Oct., 1988 | Gabel et al.
| |
4815414 | Mar., 1989 | Duffy et al.
| |
4835819 | Jun., 1989 | Duffy et al.
| |
4837090 | Jun., 1989 | Hyner et al.
| |
4842890 | Jun., 1989 | Sessa et al.
| |
4869921 | Sep., 1989 | Gabel et al.
| |
4986210 | Jan., 1991 | Hollstein et al.
| |
5078083 | Jan., 1992 | DiMaio et al.
| |
5090355 | Feb., 1992 | DiMaio et al.
| |
5259236 | Nov., 1993 | English.
| |
5306346 | Apr., 1994 | DiMaio et al.
| |
5362327 | Nov., 1994 | Sessa et al.
| |
5476689 | Dec., 1995 | Stone et al.
| |
5506491 | Apr., 1996 | Ford.
| |
5518546 | May., 1996 | Williams et al.
| |
5534064 | Jul., 1996 | Tsutsui et al.
| |
5552191 | Sep., 1996 | Horinka et al.
| |
5571323 | Nov., 1996 | Duffy et al.
| |
5607720 | Mar., 1997 | Wallace et al.
| |
5792512 | Aug., 1998 | Duffy et al.
| |
5820941 | Oct., 1998 | Felton et al.
| |
Foreign Patent Documents |
2733802 | Feb., 1979 | DE.
| |
1192210 | Sep., 1980 | JP.
| |
710852 | Jun., 1954 | GB.
| |
Other References
Reddy, Vishu, Powder Spray Technologies and Their Selection, Reprinted from
Plating and Surface Finishing, Jun. 1989.
Knobbe, Alan J., Powder Spray Guns, pp. 192-195. No date.
Hughes, J.F., Electrostatic Particle Charging: Industrial and Health Care
Applications, Research Studies Press, Ltd., 1997, pp. 1-27, 95-109 and
143-164.
Products Finishing Magazine, Jan. 1990, vol. 54, No. 4, 5 pages.
|
Primary Examiner: Parker; Fred J.
Attorney, Agent or Firm: Niro, Scavone, Haller & Niro
Claims
We claim:
1. A process for coating a selected portion of a threaded fastener with a
fluoropolymer, comprising the steps of:
supplying the fluoropolymer in powder form to a spray nozzle;
supplying high pressure gas to the spray nozzle;
discharging a stream of fluoropolymer powder entrained in the gas from the
nozzle;
subjecting the fluoropolymer powder to a triboelectric charging process so
that particles of fluoropolymer in the powder stream are triboelectrically
charged;
positioning the fastener within the powder stream to deposit a coating of
the fluoropolymer powder onto at least a substantial portion of the
threads of the fastener, whereby the triboelectric charge assists in
retaining the fluoropolymer powder on the fastener; and
heating the coated fastener to a temperature above the melting temperature
of the fluoropolymer and thereafter cooling the coated fastener to
coalesce the powder into a substantially continuous adherent fluoropolymer
coating on the fastener.
2. The coating process of claim 1, wherein the fastener is an externally
threaded fastener, and further comprising the step of removing
fluoropolymer powder deposited on portions of the fastener other than the
selected portion during or after cooling.
3. The coating process of claim 1, wherein the fastener is an externally
threaded fastener and during the heating step portions of the fastener
other than the fluoropolymer coated portion do not reach a temperature
above the melting temperature of the fluoropolymer.
4. The coating process of claim 1, wherein the threaded fastener includes a
zinc plating and wherein the zinc plating is substantially unaffected by
the heating step.
5. The coating process of claim 1, wherein the fluoropolymer powder is
charged to between about 1.times.10.sup.-7 to about 3.times.10.sup.-3
coulombs per kilogram.
6. A The process of claim 1, wherein heating of the fastener is
accomplished using induction coils.
7. The process of claim 1, wherein the heating step is accomplished in
about 1 minute or less.
8. The process of claim 1, wherein the heating step is accomplished in
about 10 seconds or less.
9. A process for coating a selected portion of an internally threaded
fastener with a fluoropolymer, comprising the steps of:
supplying the fluoropolymer in powder form to a spray nozzle;
supplying high pressure gas to the spray nozzle;
discharging a stream of fluoropolymer powder entrained in the gas from the
nozzle;
subjecting the fluoropolymer powder to a triboelectric charging process so
that particles of fluoropolymer in the powder stream are triboelectrically
charged;
positioning the fastener within the gas entrained powder stream to deposit
a coating of the fluoropolymer powder onto at least a substantial portion
of the threads of the fastener, whereby the triboelectric charge assists
in retaining the fluoropolymer powder on the fastener; and
heating the coated fastener to a temperature above the melting temperature
of the fluoropolymer and thereafter cooling the coated fastener to
coalesce the powder into a substantially continuous adherent fluoropolymer
coating on the fastener.
Description
BACKGROUND OF THE INVENTION
The present invention relates to fluoropolymer coated fasteners, and, more
particularly, to a new process for effectively and efficiently coating
preselected portions of threaded fasteners with a fluoropolymer.
It has been recognized for some time that threaded fasteners may be
protected from thread contaminants by coating the threads with
fluoropolymer resin. Typical contaminants that may interfere with proper
threaded coupling of the fasteners include paint, anti-corrosion primers,
weld spatter and solder. Coating the fastener threads with a fluoropolymer
before exposure to these contaminants, reduces or prevents the
contaminants from adhering to the fastener. In the use of such
fluoropolymer coatings, however, it is important, and often critical, that
the fluoropolymer coating be applied only to selected portions of the
fastener. Indiscriminate application of the coating over all areas of the
fastener is to be avoided. Examples of prior art teachings in this field
are found in U.S. Pat. Nos. RE33,766 and 5,221,170. The disclosures of
these patents are incorporated herein by reference.
Although the processes and coated fasteners as disclosed in the above
identified patents have achieved substantial commercial success, they
nonetheless suffer from certain disadvantages. For example, in the
practice of this prior art the fasteners are heated prior to application
of the fluoropolymer powder. As a result, the fasteners are necessarily
heated to a temperature substantially above the fluoropolymer melting
point to accommodate some cooling of the fastener during transit from the
heating station to the powder spray station. This elevated temperature, in
the range of about 750.degree. to 900.degree. F., can damage certain
fastener materials or platings, thus, limiting the applicability of the
prior art technology.
Another disadvantage associated with the prior art is that relatively large
amounts of fluoropolymer powder are required to achieve a generally
uniform and continuous coating, thereby raising the cost of the process.
Another disadvantage with the prior art is that, traditionally,
fluoropolymer coatings are baked and sintered for extended periods of
time, increasing processing time.
Initial experiments were conducted some time ago in an attempt to
electrostatically deposit fluoropolymer powders using conventional corona
charging techniques. However, the resulting fluoropolymer powder coating
was indiscriminately applied onto a wide area of the fastener, requiring
some form of masking to limit the coating to only the preselected areas
where the coating was desired. Additionally, when attempting to coat
internally threaded fasteners, Faraday cage effects come into play, which
further limits the integrity of the resulting coating. The possibility of
electrostatically depositing the powder by corona charging techniques was
therefore rejected since masking would prove too difficult and costly in
high volume production.
There is, accordingly, a need for a new fluoropolymer coating process that
employs lower temperatures, less fluoropolymer resin and is less costly;
while maintaining the benefits and advantages of the known powdered
fluoropolymer application technology.
SUMMARY OF THE INVENTION
The present invention is directed to a process for the application of
fluoropolymer to a preselected area of a threaded fastener, and
particularly to substantially all of the threads of the fastener.
The fluoropolymer is supplied to a spray nozzle in powder form and is
subjected to a triboelectrostatic charging process so that individual
particles discharged from the spray nozzle are electrically charged. In
the preferred form of the invention, the fluoropolymer powder is
triboelectrically charged, entrained in an air stream discharged from the
nozzle and directed onto the preselected area of the fastener. In this
manner a generally uniform powder coating is deposited onto the
preselected area of the fastener while the fastener is maintained at room
temperature. Thereafter, the fastener is heated to a temperature above the
melting point of the fluoropolymer to thereby coalesce the deposited
powder into a continuous film coating which adheres, upon cooling, to the
pre-selected area of the fastener.
The process of the present invention may be used with either internally or
externally threaded articles, such as internally or externally threaded
fasteners. In accordance with one preferred embodiment, an externally
threaded fastener is heated in a manner which raises the temperature of
only a preselected area of the fastener to the fluoropolymer melting
point. This preferred heating technique minimizes the retention of
fluoropolymer inadvertently deposited on areas of the fastener other than
the preselected area, and allows this undesired fluoropolymer to be easily
removed, even after heating.
Using the present invention, the coating of internally threaded fasteners
may be confined to the threaded area only and, therefore, the entire
fastener may be heated to coalesce the deposited powder.
With the present invention, heating times required for fluoropolymer
adherence may be substantially decreased.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features which are characteristic of the invention are set forth
in the appended claims. The invention itself, however, together with
further objects and attendant advantages thereof, will be best understood
by reference to the following description taken in connection with the
accompanying drawings, in which:
FIG. 1 is a plan view, illustrating a carousel assembly suitable for
implementing the process of the present invention with externally threaded
fasteners;
FIG. 2 is a partial perspective view of the assembly illustrated in FIG. 1;
FIG. 3 is a partial cross-sectional view of the fastener rotation
mechanism;
FIGS. 4 and 5 are top and side views, respectively, of an appropriate
fastener centering mechanism used in the carousel assembly illustrated in
FIG. 1;
FIG. 6 is a perspective view illustrating details of the powder stream
nozzle, fastener and fastener support, and vacuum nozzle used in the
assembly of FIG. 1; and
FIG. 7 is a partial cross-sectional view illustrating the positional
relationship between the fastener and heating coils as preferably used in
the assembly of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The process of the present invention is illustrated in FIGS. 1, 2, 6 and 7
with respect to the selective fluoropolymer coating of externally threaded
fasteners, such as a conventional weld stud. The invention is not limited,
however, to the illustrated fastener; but, rather, finds application with
both externally and internally threaded fasteners of all kinds and
configurations. Its advantages arise from the ability to easily and
expeditiously coat only preselected areas of the fastener, at high
production volumes, without the need to mask the remaining areas where the
coating is neither needed nor desired.
In FIG. 1, the fluoropolymer powder is provided to the supply port of a
conventional powder spray nozzle 10. Typical spray nozzles of this sort
employ high pressure air at about 40 to 80 psi to aspirate the supply
powder and to generate powder stream entrained in the discharging air.
Preferably, the fluoropolymer powder is a perfluoro alkoxy resin,
manufactured by DuPont under the trade designation PFA powder-white,
product code 532-5100. This powder has a particle size of about 20.+-.3
microns.
A variety of powder spray nozzles and associated supply apparatus may be
used in the practice of the present invention. Suitable examples are
disclosed in U.S. Pat. Nos. 3,579,684; 4,815,414; 4,835,819; 5,090,355;
5,571,323; and 5,792,512 whose disclosures are incorporated herein by
reference.
The fasteners may be positioned within, or conveyed to intersect, the
powder stream using well known apparatus. Again, suitable examples are
illustrated in U.S. Pat. Nos. 3,894,509; 4,120,993; 4,775,555; 4,842,890;
and 5,078,083. These patents' disclosures are also incorporated herein by
reference. The illustrated apparatus comprises a horizontally rotating
carousel 12 having fastener carrying posts 14 disposed about its
circumference. The carrying posts 14 are preferably constructed from a
material having a relatively high heat transfer coefficient, such as
aluminum, brass, steel or copper. In addition, the posts each preferably
house a centrally disposed magnet 15 to assist in maintaining each
fastener in proper position.
Each fastener carrying post 14 is rotationally mounted to the carousel 12
and may be driven by a gear or sprocket 16 extending from the lower end of
the posts. The gear will rotate when it traverses and engages an
appropriately positioned, variable speed, motor-driven timing belt (not
shown), thereby rotating the post and fastener when the fastener is in the
powder stream. Examples of other suitable rotational fastener carriers are
disclosed in U.S. Pat. Nos. 4,842,890; 5,078,083 and 5,090,355 whose
disclosures are incorporated herein by reference.
A fastener centering station 20 may also be employed. This device centers
the fasteners on carrying post 14 to provide wobble-free rotation when the
fastener is in the powder stream. One preferred form of this centering
station is illustrated in FIGS. 2-5.
It utilizes a fastener engaging wheel 22 which is rotationally driven via
drive post 24, drive belt 26, connected wheel 38, and a drive assembly 28
including a drive belt 18. Belt 18 engages sprockets 16 to rotate the
fasteners. Belt 26 may be driven by the same or a second, suitably
positioned, variable speed motor (not shown). The radial position of wheel
22 relative to carousel 12 is made adjustable by mounting the drive post
24 on a pivotally mounted support bar 30. The bar 30, in turn, can be
positioned using threaded rod 32. Rotation of rod 32 will pivot support
bar 30, thereby adjusting the radial position of wheel 22.
In accordance with the preferred embodiment of the invention, the powder
stream may be configured or shaped, at least in part, by the geometry of
the nozzle discharge port. Thus, a vertically narrow stream may be formed
with a nozzle having a small vertical dimension and, conversely, a
vertically broad stream will result from use of a nozzle having a large
vertical dimension. The horizontal extent of the stream may be similarly
controlled. In addition, an air knife 40 (see FIG. 6) can be positioned
either below or above (or both below and above) the nozzle 10. As
illustrated, the air knife 40 positioned below the nozzle discharge port
will delimit the lower extent of the powder stream, tending to reduce the
deposition of powder onto the fastener's lower area or the fastener
carrying posts 14.
It is also desirable to employ a vacuum collection system to capture and
re-circulate powder from the powder stream that is not deposited on the
fasteners. Typically, the vacuum nozzle 42 will be located, as
illustrated, in juxtaposition to the spray nozzle 10 and will be sized
somewhat larger than the cross-sectional area of the powder stream.
In accordance with an important aspect of the invention, it is necessary to
condition the fluoropolymer powder so that it will be retained on only a
preselected area of the fastener, usually substantially all of the
threaded portion of the fastener. The powder must be evenly deposited onto
the preselected area and retained until heated to its melting point and
thereby coalesced into an adherent continuous coating. Moreover, it must
be so retained while the fastener is transported, via the carousel 12 or
other conveyor, to the heating station. Preferably, the powder is
triboelectrically charged by its rapid passage through appropriate tubing
from the powder supply reservoir and by its rapid passage through the
spray nozzle itself. In this way, a moderate electrostatic charge, in the
range of about 1.times.10.sup.-7 to about 3.times.10.sup.-3 coulombs per
kilogram, will be generated on the powder stream.
Although nylon, vinyl or polyester tubing is preferred, other materials,
even electrically conductive tubing such as metal has also found to
perform satisfactorily. An electrical charge, or Mass Charge Density, on
the powder in the range of about 1.times.10.sup.-3 to 3.times.10.sup.-3
coulombs per kilogram has been found to work well, and this charge may be
generated using a conventional copper spray nozzle with air velocity
through the nozzle in the range of about 300 to 350 meters per second and
powder flow rates of about 1.5 to 3.0.times.10.sup.-4 kilograms per
second.
It has been found that the coverage of a triboelectric charged particle
coating is defined mainly by the direction of the entraining air volume
and not by corona field effects. In other words, the triboelectric charge
assists in retaining the fluoropolymer on the areas of the fastener that
directly intersect the powder stream while the shape of the powder stream
and the use of an appropriately positioned air knife minimize the
deposition of powder on other areas of the fastener where a fluoropolymer
adherent coating is undesirable. Thus, by properly configuring the powder
stream and positioning the fasteners relative to the stream, a suitable
fluoropolymer powder coating may be deposited substantially on only the
desired areas of the fasteners. As one example, the coating of internally
threaded fasteners may be confined to the threaded area only and,
therefore, the entire fastener may be heated to coalesce the deposited
powder.
It has also been discovered that the use of triboelectrically charged
powder results in a highly uniform and complete powder coating with a
minimum volume of powder. Indeed, very uniform and pinhole free coatings
are achieved, after heating, even with coatings that are less than 1/2 mil
(0.0005 in) in thickness.
After the fasteners have been coated with fluoropolymer powder, they are
transported via the carousel 12 into a heating station. Again, many
different heating apparatus may be employed, but an induction heating coil
44 has been found most satisfactory. Such coils are described in U.S. Pat.
Nos. 5,306,346 and 5,632,327; whose disclosures are incorporated herein by
reference. Induction heating raises the temperature of the fastener at the
fastener's surface. Because the fluoropolymer is in direct contact with
this surface, it is heated via conductive heat transfer. As a result, the
fastener need only be heated slightly above the fluoropolymer melting
point (about 580.degree. F.), or typically in the range of about
600.degree. to 650.degree. F. This is substantially below the temperatures
required for preheated fastener fluoropolymer coating which typically
requires heating of the fasteners to about 750.degree. to 900.degree. F.
Consequently, the process of the present invention finds particularly
advantageous application when coating plated fasteners, such as zinc
plated fasteners which will often degrade when heated above about
700.degree. F.
According to a preferred aspect of the present invention, the
fluropolymer-coated fasteners are heated for a relatively short period of
time, sufficient to melt the fluoropolymer. Using induction heating coils,
the fluoropolymer powder, initially at room temperature, is quickly heated
to temperatures which may be in the range of 600.degree. F.-650.degree. F.
Thus, with the present invention, heating times required for application
of the fluoropolymer powder may be substantially lessened, such as to 30
minutes or less. Preferably, heating times are only 5-10 minutes or less
and, still more preferably, are less than about 1 minute. In the
particularly preferred embodiment, melting of the fluoropolymer coating on
the desired portions of the fastener is accomplished in less than about 10
seconds, and even as fast as about 1-2seconds or less.
In one preferred embodiment using the apparatus shown in the drawings, M10
weld studs were coated. The number of fasteners coated and the time taken
to achieve melting of the fluoropolymer powder for each fastener is shown
below:
______________________________________
Number of fasteners coated/minute
Seconds to achieve melting
______________________________________
60 9.6
120 4.8
180 3.2
240 2.4
______________________________________
In accordance with the present invention, the induction heating coils 44
can be positioned to selectively heat the fasteners. As illustrated in
FIG. 7, the weld studs are supported on the carrying posts 14 so that
their threaded shank portions pass directly between the coils 44 while
their heads are positioned below the coils. In this way, the threaded
portions will be heated to the desired temperature while the non-threaded
portions will remain below the fluoropolymer melting point. This selective
heating is facilitated by using a highly heat conductive carrying post and
magnet which act as a heat sink to minimize the temperature of the
fastener adjacent the post.
Selective heating has several advantages. First, it insures that the
adherent fluoropolymer coating is achieved only in the areas where
fluoropolymer melting point temperatures are reached--in the threaded
portion. Thus, any fluoropolymer powder deposited in other areas will be
easily removed when the fastener is submerged in the anti-corrosion
cooling bath. Moreover, lower energy consumption and higher production
rates may also be achieved. Finally, selective heating allows the use of
less discriminating powder application techniques, such as corona charging
electrostatic deposition of the fluoropolymer, where powder is initially
deposited over substantially greater areas of the fastener than are
desired for the finally coated part.
It should be noted here, that references to a powder "deposited" on and
"retained" on the fastener are intended to mean only that the powder will
remain in place during transport to the heating station. In this
condition, it can be easily removed from the fastener via high velocity
gas streams, mechanical brushing or a liquid wash. On the other hand,
references to an "adherent" coating are intended to mean that the
fluoropolymer has coalesced into a substantially continuous film that
adheres to the fastener's surface even when exposed to high velocity air
or liquid streams or moderate mechanical abrasion. Most preferably,
however, even the "adherent" fluoropolymer coating will be dislodged from
the threaded portions of the fastener when engaged by a mating fastener
and subjected to appropriate clamping loads.
After the fasteners pass through the heating station, they are removed from
the carrying posts by a suitable cam 46 and/or air streams and either air
cooled or immersed in a cooling bath, typically an aqueous based
anticorrosion bath or other liquid treatment. The fasteners may be air
cooled for about the same time as they are heated, prior to immersion in
the cooling bath.
The resulting coated fastener has a fluoropolymer film adherent to its
threaded portion. The film is generally uniform in thickness both at the
crests and roots of the threads and is substantially pinhole free.
Moreover, it is a substantially pure fluoropolymer coating having no
binders, fillers or other incorporated compounds. In accordance with the
present invention, the film may contain over 98% fluoropolymer, the
remainder being a coloring pigment such as titanium dioxide. If desired,
however, other compounds can be added to enhance the coating's mechanical
and/or chemical properties.
The process of the present invention permits the selective coating of
relatively small threaded fasteners at high production volumes without the
need for preapplied masks on portions of the fastener where no coating is
desired.
Of course, it should be understood that various changes and modifications
to the preferred embodiments described herein will be apparent to those
skilled in the art. Such changes and modifications can be made without
diminishing its attendant advantages. It is therefore intended that such
changes and modifications be covered by the following claims:
Top